1. Field of the Invention
The present invention relates to a novel polyimide copolymer, and more particularly a novel polyimide copolymer with a distinguished heat-resistant dimensional stability.
2. Related Art
It is known that available polyimide resins up to now have distinguished characteristics such as good heat resistance, electrical and mechanical properties, etc., but generally suffer from high coefficient of linear expansion, high coefficient of humidity expansion, and poor dimensional stability. Particularly, when the coefficient of linear expansion is not less than 30 ppm/xc2x0 C., polyimide resins will have a poor heat-resistant dimensional stability, and thus have problems such as generation of warping or curling when applied to lamination with a metal foil to form a flexible wiring substrate, etc.
To solve these problems, polyimide films obtained, for example, from pyromellitic acid dianhydride and 6-amino-2-(p-aminophenyl)benzimidazole through polyamic acid have been proposed. They have a lower coefficient of linear expansion, but are very brittle, and thus are impractical for use as films.
A kind of polyimide copolymers based on 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid dianhydride-6-amino-2-(p-aminophenyl)benzimidazole has been already proposed by the present applicant (WO 01/29136), where isopropylidene bis(4-phenyleneoxy-4-phthalic acid)dianhydride is used together with 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid dianhydride, and polyimide copolymers per se obtained by polycondensation of these three components have a high adhesive strength and can produce a metal laminate having a satisfactory peel strength, even when bonded to a metal foil without any adhesive layer, and furthermore have a distinguished solder heat resistance, but fail to satisfy the desired heat-resistant dimensional stability.
An object of the present invention is to provide an aromatic tetracarboxylic acid dianhydride-6-amino-2-(p-aminophenyl)benzimidazole-based polyimide copolymer with a good heat-resistant dimensional stability without any deterioration of mechanical properties inherent in the polyimide resin itself when used as a film.
Such an object of the present invention can be attained by a novel film-formable polyimide copolymer, which comprises two kinds of tetracarboxylic acid dianlydrides, consisting of (A) pyromellitic acid dianhydride and (B) 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid dianhydride, and (C) 6-amino-2-(p-aminophenyl)benzimidazole.
A mixture of (A) pyromellitic acid dianhydride (benzene-1,2,4,5-tetracarboxylic acid dianhydride) and (B) 3,3xe2x80x2,4,4xe2x80x2-benzophenonetetracarboxylic acid dianhydride, as given by the following structural formula: 
is used as tetracarboxylic dianhydride components for the present novel polyimide copolymer.
Components (A) and (B) are used in a proportion of Component (A) to Component (B) of about 5-about 90 mol. % to about 95-about 10 mol. %, preferably about 10-about 80 mol. % to about 90-about 20 mol. %, sum total being 100 mol. %. Below about 10 mol. % of Component (B), the film to be formed from the resulting copolymer will be more brittle, whereas above about 95 mol. % of Component (B) the content of Component (A) will be correspondingly decreased, so that no effective improvement of heat-resistant dimensional stability will be attained.
As a diamine capable of reacting with these two tetracarboxylic acid dianhydrides e.g. Component (C), 6-amino-2-(p-aminophenyl)benzimidazole, as given by the following structural formula: 
is used.
Reaction of tetracarboxylic acid dianhydrides with the diamine is carried out not only in an aprotic polar solvent such as dimethyl formamide, dimethyl acetamide, N-methyl-2-pyrrolidone, etc., but also in a polar solvent such as m-cresol. Practically, a polar solvent solution of diamine is dropwise added to a polar solvent solution of tetracarboxylic acid dianhydrides at a temperature of about 0xc2x0-about 30xc2x0 C. with stirring, then to conduct reaction at about 0xc2x0-about 60xc2x0 C. with stirring for about 0.5-about 5 hours after the dropwise addition. It seems that polyamic acid as a polyimide precursor copolymer can be formed thereby.
The resulting solution containing polyamic acid thus formed is cast onto a substrate such as a metallic foil, etc., and dried with hot air at about 60xc2x0-about 200xc2x0 C., preferably about 80xc2x0-about 180xc2x0 C., for about 5-about 100 minutes. Then, the resulting polyamic acid film is peeled away from the substrate and heated at about 150xc2x0-about 400xc2x0 C., preferably about 200xc2x0-about 400xc2x0 C. for 3 minutes to about 2 hours, thereby obtaining a polyimide copolymer film. Even by heating the polyamic acid film as laid on the metallic foil at a temperature capable of conducting polyimidization reaction without peeling the polyamic acid film away from the metallic foil, a polyimide copolymer laminated metallic foil can be directly obtained.
By reaction of (A) tracarboxylic acid dianhydride with (C) diamine, a polyimide copolymer having the following repeat unit can be obtained: 
By reaction of (B) tracarboxylic acid dianhydride with (C) diamine, a polyimide copolymer having the following repeat unit can be obtained in addition to the aforementioned repeat unit: 
Polyimide copolymers having these repeat units are insoluble in so far available solvents and thus their molecular weights or viscosities cannot be determined or their ranges cannot be specified, but the polyimide copolymers must surely have film-formable molecular weights. At the stage of polyamic acid formation, where the polyimide precursor copolymer seems to be formed, it is possible to determine the viscosity of a reaction mixture solution having a given solid concentration, though the viscosity depends on reaction time, etc.
The present novel polyimide copolymer has a film formability and shows a distinguished heat-resistant dimensional stability without any deterioration of mechanical properties, etc. inherent to the polyimide resins per se, when used as films. The present polyimide copolymer films having these characteristics will not produce warping or curling when used in flexible wiring substrates, etc. requiring a higher dimensional stability, upon lamination with a metallic foil as a base film or a cover film.
The present invention will be described below, referring to Examples.